German Offenlegungsschrift No. 31 23 760 discloses a yarn storing, feeding and measuring device for jet looms having a stationary storage drum onto which an intermediate yarn store is wound by a winding-on device and from which the yarn is withdrawn spiralling around the withdrawal end of the storage drum, yarn sensing means arranged such that the yarn is passing its detection area during withdrawal from the drum, said yarn sensing means producing pulse signals, each pulse signal indicating that the yarn is passing a detection area of the yarn sensing means, a plurality of yarn stopping devices arranged at angular intervals around the storage drum, said yarn stopping devices consisting of yarn stopping elements and actuator means moving said stopping elements into and out of the path of the yarn being withdrawn, and an actuator control device adjustable to a desired yarn length to be withdrawn, said control device being responsive to said pulse signals in such a way that an actuating signal is transmitted to a selected yarn stopping device whose angular position corresponds to the position of the yarn when said desired yarn length has been withdrawn. The yarn sensing means of this prior art device consists in a plurality of yarn sensors, each of said sensors being associated with a yarn stopping device. Hence, the number of yarn sensors required for such a prior art device corresponds to the number of yarn stopping devices. Yarn storing, feeding and measuring devices of the above mentioned kind not only serve to intermediately store the weft yarn on a storage drum, but also serve to supply the jet loom with a weft yarn having a desired length. For the latter purpose, this prior art device carries out the following steps in order to obtain the desired yarn length for each weft yarn insertion: After releasing or deactuating the yarn stopping device actuated at the end of a previous yarn withdrawal cycle the yarn is withdrawn spiralling around the withdrawal end of the storage drum. Thereby, the yarn repeatedly passes the detection areas of the plurality of yarn sensors arranged at the withdrawal end of the storage drum in a spaced, angular relationship with respect to each other. Each yarn sensor generates a pulse signal indicating that the yarn passes its detection area, these pulse signals being fed to the control device. Hence, the control device receives a number of pulse signals, this number corresponding to the number of yarn sensors being passed by the yarn during the withdrawal. By counting the pulse signals received from the yarn sensor the control device generates a count value corresponding to the actual position of the withdrawal point of the yarn with respect to the yarn sensors. The count value corresponds to the length of yarn withdrawn from the storage drum. When the count value corresponds to the desired yarn length to be withdrawn, the control device actuates the stopping device located with respect to the angular movement of the withdrawal point of the yarn behind the yarn sensor which generated the last pulse signal. Thereby, the withdrawal of the yarn is stopped so that the desired yarn length is obtained. This prior art device is costly and complicated due to the large number of yarn stopping sensors required to achieve a sufficiently large number of different yarn lengths. Furthermore, it is undesirable to have a large number of yarn sensors due to the fact that such yarn storing, feeding and measuring devices have to be designed as compact as possible so as to meet with the technical requirements of the customers and users of such devices. A further drawback is caused by the fact that yarn sensors are also sensitive in operation, since they usually comprise optical elements which can be covered by lint. If one of the plurality of yarn sensors is covered by lint, it will no longer generate pulse signals when the yarn passes its detection area resulting in a wrong count value in the control device. Hence, the respective yarn length of the inserted weft yarn will become greater than the desired yarn length.
German Offenlegungsschrift No. 31 23 760 also discloses a yarn storing, feeding and measuring device using only one yarn sensor for detecting the withdrawal of one complete turn of yarn at the withdrawal end of the drum. In order to be able to adjust the yarn length to be withdrawn, this prior art device makes use of a drum whose diameter can be mechanically varied. The same concept is disclosed in French Publication No. 2 166 332 and PCT Published Application No. 82/04446. A mechanical adjustment of the diameter of the drum calls for complicated mechanical means which makes the device costly and liable to malfunctions.
The present invention seeks to improve the device of the above mentioned type so as to give it a simpler, cost-saving structure and a more reliable function without giving up the advantage of adjustability of the yarn length to be withdrawn.